A brake unit of the kind defined above is shown in German Patent Office Publication No. 25 13 870. In this case the wedge surface of the wedge element has a constant wedge angle not only for the brake operation stroke but also for the brake application stroke. The leverage is in other words constant over the whole working range of the wedge element. The brake application stroke will thus not be faster than the brake operation stroke.
One solution to the problem of attaining a faster brake application stroke is disclosed in German Patent Office Publication No. 26 53 320. In this case there are two wedge surfaces on the wedge element: a first wedge surface with a comparatively great wedge angle for a fast application stroke and a second wedge surface with a comparatively small wedge angle for a slower and more powerful brake operation stroke.
This solution, however, has certain drawbacks, of which one is the comparatively great width required for the wedge element due to the two wedge surfaces and another one is the abrupt transition from one wedge angle to another. This latter disadvantage means that it may happen, for example if a built in slack adjuster does not work quite properly, that the application stroke is consumed, before the transition point is reached.